This invention relates generally to a pressurized shock absorber and relates particularly to a pressurized and degassed dual tube hydraulic shock absorber having a gas cell.
In a conventional type of hydraulic shock absorber, a piston is carried on the end of a reciprocating rod that extends from the shock absorber cylinder and is connected to one part of a movable mass, such as the chassis of a motor vehicle being the typical use configuration, or alternatively to the vehicle running gear for inverted mounting of the shock absorber. The piston reciprocates in the cylinder which is in flow communication with a reservoir for hydraulic fluid displaced from the cylinder, the cylinder and reservoir structure usually being connected to another movable mass, such as the running gear of a motor vehicle or alternatively to the vehicle chassis for inverted mounting. In the dual tube type of shock absorber, a cylindrical body tube surrounds the cylinder. The annular space between the body tube and the cylinder forms a reservoir for hydraulic fluid. A volume of fluid equal to the displacement of the rod on which the piston is mounted is displaced from the shock absorber cylinder through suitable resistance valves in the piston and in the base of the cylinder into the reservoir during the compression stroke of the shock absorber. On the rebound stroke, the volume of fluid that was displaced from the shock absorber cylinder during the compression stroke is returned to the shock absorber cylinder from the reservoir through a low resistance valve to refill the cylinder. Examples of dual tube hydraulic shock absorbers are disclosed in U.S. Pat. No. 3,763,970.
To provide compensating space for the pulsing action of the hydraulic fluid between the shock absorber cylinder and the reservoir, a volume of gas is retained in the reservoir. However, this gas volume is preferably isolated from contact with the hydraulic fluid, since pulsing flow of hydraulic fluid between the shock absorber cylinder and the reservoir causes a high degree of turbulance of the fluid in the reservoir with the result that the hydraulic fluid picks up air in the reservoir and becomes aerated to such an extent as to cause a disturbing reduction in shock dampening capacity of the shock absorber.
To avoid this aeration effect in the hydraulic fluid, deformable gas bags or cells are placed within the reservoir to isolate the hydraulic fluid within the shock absorber from gas contact, the shock absorber being completely filled with hydraulic fluid with the exception of the closed gas cell . This cell contains a predetermined volume of a selected gas. The volume of the selected gas in the sealed cell is such that under conditions of full collapse of the shock absorber (full compression stroke) at the highest temperature expected in normal operation, the cell will not be fully collapsed. Thus, there will always be a gas volume in the reservoir chamber to accommodate liquid displaced from the shock absorber cylinder. The gas volume in the cell is also such that when the shock absorber is fully extended at the lowest temperature at which it normally operates, the expansion of the gas in the cell will still sufficiently displace the hydraulic fluid to insure filling of all voids in the shock absorber. Also, the cell tends to compensate for normal expansion and contraction of the hydraulic fluid during operation of the shock absorber in various ambient temperatures.
Thus, with the gas in the reservoir chamber completely isolated from the hydraulic fluid, there will be no absorption of the gas into the hydraulic fluid, and aeration of the hydraulic fluid is eliminated. Representatively, dual tube, gas cell hydraulic shock absorbers are disclosed in U.S. Pat. No. 2,997,291; 3,123,347; and 3,024,875 hereby incorporated by reference.
Notwithstanding the use of gas cells, there remains a problem in removing small amounts of residual air remaining after filling a gas cell shock absorber with hydraulic fluid thereby preventing full benefit of use of the gas cell. As representatively shown in U.S. Pat. No. 3,024,875 cited above, a relatively small amount of free air in the reservoir, e.g. 2 to 4% by volume, has a substantial effect on the response lag of the shock absorber.
Additionally, there is a problem with air ingression during the useful life of the shock absorber as hydraulic fluid gradually leaks in the seal area around the piston rod where the rod enters the shock absorber. Gas cell materials have been used which tend to absorb air dissolved in the hydraulic fluid. However, during relatively short periods of extreme operation, this selective absorption is not effective.